Steering system

ABSTRACT

A steering controller for an electric power steering device  110  includes a base signal computing part  51  for computing a base signal D T  in accordance with at least the steering torque; a damper compensation signal computing part  52  for computing a damper compensation signal in accordance with an angular velocity of an electric motor  4  or a speed of steering wheel turn; and an inertia compensation signal computing part  53  for compensating inertia and viscosity in the steering unit. The electric motor is driven by a target signal IM 1  obtained by compensating the base signal with a damper compensation signal and an inertia compensation signal, to provide a steering auxiliary force. The target signal of the auxiliary torque is compensated so that a difference between a reference self-aligning torque of front wheel in a front wheel steering vehicle and a self-aligning torque of front wheel in an all-wheel steering vehicle is provided to a driver as a responsive feeling from the steering torque.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the foreign priority benefit under Title 35,United States Code, section 119 (a)-(d), of Japanese Patent ApplicationNo. 2007-036696, filed on Feb. 16, 2007 in the Japan Patent Office, thedisclosure of which is herein incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a steering system in which an operationof toe angle of rear wheels is controlled based on a turning angle offront wheels and a vehicle speed, and particularly to a steering systemin combination with an electric power steering device which assistssteering wheel turn of the front wheels.

2. Description of the Related Art

An electric power steering device is a device in which an electric motorgenerates an auxiliary torque in accordance with a magnitude of asteering torque, and the auxiliary torque is transmitted to a steeringunit, to thereby reduce a steering effort required by a driver forsteering. There is disclosed a technique in which a base current (assisttorque) determined by a steering torque and a vehicle speed iscompensated by inertia and damping (viscosity) in the steering unit, andthe compensated current is used as a target current for controlling anelectric motor (see Japanese patent application unexamined publicationsNo. 2002-59855 (FIG. 2) and No. 2000-177615 (FIG. 2)).

As an electric motor for such an electric power steering device, abrushless motor is disclosed in Japanese patent application unexaminedpublication No. 2004-322814 (or corresponding U.S. Pat. No. 6,926,299)(FIGS. 2 and 3).

Also in Japanese patent application examined publication H6-47388 (FIG.2), there is disclosed an all-wheel independent steering device in whichoperation of all running wheels are individually controlled based on anoperation angle of steering wheels and a vehicle speed.

In Japanese patent application unexamined publication No. 2002-59855,properties including a base current, damping and inertia are computedusing a base table, a damper table and an inertia table whichsubstantially has a differential property, respectively. Herein, settingof each table, which includes functions of steering torque, vehiclespeed and electric motor angular velocity, will be discussed. The basetable is set in such a manner that a driver is provided with roadinformation and a steady responsive feeling from a steering torque, inaccordance with an increase in the vehicle speed, and thus it isrequired that a gain be made lower when the vehicle speed is higher, andthat a dead zone is set larger for giving a larger manual steering zone.The base table is also set so as to give an excellent steering feeling,and therefore, it is required that a response lag, which may otherwisebe caused by electric motor inertia, viscosity or the like, be reducedby using the inertia table.

The inertia table is set so as to improve vehicle properties byimproving response of the steering wheel and also convergence of thesteering wheel position, by cooperating with a steering damper effect.Therefore, it is required that the inertia table be substantiallyprovided with a differential property based on the steering torque, tothereby increase or decrease an assist on the electric motor inaccordance with the changed portion of the steering torque, i.e., arotational acceleration (steering rotational acceleration) of theelectric motor.

However, in a case of a steering system in which a toe angle changerwhich controls an operation of a toe angle of rear wheel, based on anoperation angle of a steering wheel or a turning angle of front wheel aswell as a vehicle speed, is combined with an electric power steeringdevice, a responsive feeling from a steering torque given by theelectric power steering device of the vehicle having the above-mentionedsteering system may bring discomfort to a driver who is used to aresponsive feeling from the steering torque given by an electric powersteering device of a vehicle that has only a front wheel steeringfunction.

In addition, even when the toe angle changer is in an abnormal state(for example, the toe angle of the rear wheel is locked) or when thesteering of the front wheel is assisted by the electric power steeringdevice, the driver may not feel any anomaly in the responsive feelingfrom the steering torque, and may keep operating the steering wheel inthe same manner.

Therefore, it would be desirable to provide a steering system thatsolves the above-mentioned problems.

SUMMARY OF THE INVENTION

In one aspect of the present invention, there is provided a steeringsystem including: an electric power steering device which includes asteering unit of front wheels having an electric motor configured togenerate an auxiliary torque in accordance with at least a steeringtorque, and is configured to transmit the auxiliary torque to thesteering unit; toe angle changers capable of changing toe angles ofrespective right and left rear wheels in accordance with at least aturning angle of the front wheels and a vehicle speed; and a steeringcontroller configured to control the electric power steering device andthe toe angle changer, the steering controller including: an auxiliarytorque calculating unit configured to calculate a target value of theauxiliary torque and to output a target signal for driving the electricmotor, in which a difference between a first self-aligning torquegenerated at the front wheels and a second self-aligning torquegenerated at front wheels of a hypothetical vehicle having only a frontwheel steering function is compensated.

According to this steering system, the steering controller cancompensate the target signal of the auxiliary torque on the front wheelsteering, in such a manner that a responsive feeling from the steeringtorque on the steering wheel of a vehicle, during turning motion orlateral-directional motion with activation of the toe angle changer,becomes the same as the responsive feeling from the steering torque inthe case of a vehicle having only a front wheel steering function.

It is preferable in the steering system that the steering controllerfurther includes a restoring torque calculating unit configured tocalculate the first self-aligning torque based on at least a yaw rate,speed and slip angle of the vehicle and the turning angle of the frontwheels, a reference restoring torque calculating unit configured tocalculate the second self-aligning torque based on at least the vehiclespeed and the turning angle of the front wheels, and a differencecompensation unit configured to calculate a difference between the firstself-aligning torque and the second self-aligning torque and tocompensate the target signal with the difference.

According to this feature, the first self-aligning torque can becalculated in the restoring torque calculating unit, based on at leastthe yaw rate, the vehicle speed, the slip angle and the turning angle ofthe front wheel of the vehicle; a second self-aligning torque generatedat the front wheel in the case of a vehicle having only a front wheelsteering function can be calculated in the reference restoring torquecalculating unit, based on at least the vehicle speed and the turningangle of the front wheel of the vehicle; and the target signal can becompensated based on a difference between the first self-aligning torqueand the second self-aligning torque.

In another aspect of the present invention, there is provided a steeringsystem including: an electric power steering device which includes asteering unit of front wheels having an electric motor configured togenerate an auxiliary torque in accordance with at least a steeringtorque, and is configured to transmit the auxiliary torque to thesteering unit; toe angle changers capable of changing toe angles ofrespective right and left rear wheels in accordance with at least aturning angle of the front wheels and a vehicle speed; and a steeringcontroller configured to control the electric power steering device andthe toe angle changer, the steering controller including: an auxiliarytorque calculating unit configured to calculate a target value of theauxiliary torque, and an anomaly detection unit configured to detect anabnormal state of the toe angle changer, the auxiliary torquecalculating unit including a first table for calculating the auxiliarytorque in a case where the toe angle changer is in a normal sate, and asecond table for calculating the auxiliary torque in a case where thetoe angle changer is in an abnormal state, wherein, when the anomalydetection unit detects an abnormal state of the toe angle changer, theauxiliary torque calculating unit switches from the first table to thesecond table and calculates the target value which makes a responsivefeeling from the steering torque larger.

According to this steering system, when the anomaly detection unitdetects an abnormal state of the toe angle changer, the auxiliary torquecalculating unit can switch from the first table to the second table andcan calculate a target value so as to increase the responsive feelingfrom the steering torque.

It is preferable in the above-mentioned steering systems that thesteering controller includes an anomaly detection unit configured todetect an abnormal state of the toe angle changer, the auxiliary torquecalculating unit includes a first table for calculating the auxiliarytorque in a case where the toe angle changer is in a normal state, and asecond table for calculating the auxiliary torque in a case where thetoe angle changer is in an abnormal state, and when the anomalydetection unit detects an abnormal state of the toe angle changer, theauxiliary torque calculating unit switches form the first table to thesecond table and calculates the target value which makes a responsivefeeling from the steering torque larger.

According to this feature, when the anomaly detection unit detects anabnormal state of the toe angle changer, the auxiliary torquecalculating unit can switch from the first table to the second table andcan calculate a target value so as to increase the responsive feelingfrom the steering torque.

BRIEF DESCRIPTION OF THE DRAWINGS

The various aspects, other advantages and further features of thepresent invention will become more apparent by describing in detailillustrative, non-limiting embodiments thereof with reference to theaccompanying drawings.

FIG. 1 is a schematic diagram of an entire four-wheel vehicle having asteering system according to an embodiment of the present invention.

FIG. 2 is a diagram of an electric power steering device in the steeringsystem.

FIG. 3 is a plain view of a toe angle changer on a left rear wheel sidein the steering system.

FIG. 4 is a schematic cross sectional view showing a structure of anactuator of a toe angle changer.

FIG. 5 is a schematic diagram of a control function of a steeringcontrol ECU and toe angle changers in the steering system.

FIGS. 6A and 6B are graphs showing properties of a base signal computingpart and a damper compensation signal computing part, respectively.

FIG. 7 is a block configuration diagram showing detailed functions of aself-aligning torque compensation computing part.

FIG. 8 is a block configuration diagram showing a control function of atoe angle change control ECU of a toe angle changer.

FIG. 9 shows a vehicle motion in accordance with a change in anoperation angle, in a case of a vehicle to which the steering system ofthe present invention is applied: (a) shows a change over time of anoperation angle of a steering wheel, (b) shows a change over time of ayaw rate γ of the vehicle, (c) shows a change over time of a slip angleβ of the vehicle, and (d) shows a change over time of a lateralacceleration G_(S).

FIG. 10 shows a vehicle motion in accordance with a change in anoperation angle, in a case of a vehicle having only a front wheelsteering function: (a) shows a change over time of an operation angle ofa steering wheel, (b) shows a change over time of a yaw rate γ of thevehicle, (c) shows a change over time of a slip angle β of the vehicle,and (d) shows a change over time of a lateral acceleration G_(S).

FIG. 11 shows a change over time of restoring moment of two front wheelsaround kingpin axis (steering axis), in accordance with a set input ofthe operation angle of the steering wheel shown in FIGS. 9 and 10.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Embodiments of the present invention will be described in detail belowwith reference to the accompanying drawings.

Embodiments

The embodiments of the present invention will be described withreference to FIG. 1 to FIG. 8.

FIG. 1 is a schematic diagram of an entire four-wheel vehicle having asteering system according to an embodiment of the present invention.FIG. 2 is a diagram of an electric power steering device.

As shown in FIG. 1, a steering system 100 includes an electric powersteering device 110 having an electric motor 4, which is configured toassist steering of front wheels 1L, 1R by a steering wheel 3; toe anglechangers 120L, 120R configured to independently change toe angles ofrear wheels 2L, 2R by respective actuators 30, in accordance with aturning angle of the front wheels 1L, 1R by the electric power steeringdevice 110 and a vehicle speed; a steering controller 130 (hereinbelow,referred to as “steering control ECU”) configured to control theelectric power steering device 110 and the toe angle changers 120L,120R; and various sensors, including a vehicle speed sensor S_(V), a yawrate sensor S_(Y) and a lateral acceleration sensor S_(GS).

(Electric Power Steering Device)

The electric power steering device 110 includes, as shown in FIG. 2, thesteering wheel 3, a main steering shaft 3 a attached thereto, a shaft 3c and a pinion shaft 7, which shafts are connected through two universaljoints 3 b. The pinion shaft 7 has a pinion gear 7 a provided on a lowerend of the pinion shaft 7, which engages with rack teeth 8 a of a rackshaft 8 which can reciprocate in a vehicle width direction. Torespective ends of the rack shaft 8, the left front wheel 1L and theright front wheel 1R are connected through tie rods 9, 9. With thisconfiguration, the electric power steering device 110 can changetraveling direction of the vehicle by the operation of the steeringwheel 3. Herein, the rack shaft 8, the rack teeth 8 a and the tie rods9, 9 constitute a steering wheel turn mechanism.

It should be noted that the pinion shaft 7 is supported by a steeringgear box 6: an upper portion, a middle portion and a lower portion ofthe pinion shaft 7 are supported through bearings 3 d, 3 e and 3 f,respectively.

The electric power steering device 110 also has the electric motor 4 forsupplying an auxiliary steering effort to reduce a steering effortrequired at the steering wheel 3. The electric motor 4 has an outputshaft with a worm gear 5 a which engages with a worm wheel gear 5 bprovided on the pinion shaft 7.

In other words, the worm gear 5 a and the worm wheel gear 5 b constitutea deceleration mechanism. In addition, a rotor (not shown) of theelectric motor 4, and the components connected to the electric motor 4,such as the worm gear 5 a, the worm wheel gear 5 b, the pinion shaft 7,the rack shaft 8, the rack teeth 8 a and the tie rods 9, 9, constitute asteering unit.

The electric motor 4 is a three-phase brushless motor formed of a stator(not shown) with a plurality of field coils as well as the rotor whichrotates in the stator, for converting electric energy to kinetic energy(P_(M)=ωT_(M)).

Herein, ω represents an angular velocity of the electric motor 4, andT_(M) represents a torque generated at the electric motor 4. Inaddition, a relationship between the generated torque T_(M) and anoutput torque T_(M)* actually obtained as an output can be representedby the following formula (1):

T _(M) *=T _(M)−(C _(m) dθ _(m) /dt+J _(m) d ²θ_(m) /dt ²)i ²  (1)

where i represents a reduction gear ratio of the worm gear 5 a to theworm wheel gear 5 b; θ _(m) represents the rotation angle of theelectric motor; and J_(m) and C_(m) represent the inertia moment and theviscosity coefficient, respectively, of the rotor of the electric motor4.

As is apparent from the formula (1), the relationship between T_(M)* andθ_(m) can be expressed with J_(m) and C_(m) of the rotor of the electricmotor 4, which means the relationship is independent of the vehicleproperties or the vehicle state.

Herein a steering torque applied to the steering wheel 3 is representedas T_(S), and a coefficient of an assist amount A_(H) by the torquegenerated at the electric motor 4, which has been powered through thedeceleration mechanism, is represented as, for example, k_(A)(VS), whichvaries as a function of the vehicle speed VS. Since the formulaA_(H)=k_(A)(VS)×T_(S) is established in this case, a pinion torqueT_(P), which is a road load, can be represented by the following formula(2):

T _(P) =T _(S) +A _(H) =T _(S) +k _(A)(VS)×T _(S)  (2)

From this formula, the steering torque T_(s) can be represented by thefollowing formula (3).

T _(S) =T _(P)/(1+k _(A)(VS))  (3)

Therefore, the steering torque T_(S) is reduced to 1/(1+k_(A)(VS)) ofthe pinion torque T_(P) (load). For example, if k_(A)(0)=2 with thevehicle speed VS=0 km/h, the steering torque T_(S) is controlled to onethird of the pinion torque T_(P), and if k_(A)(100)=0 with the vehiclespeed VS=100 km/h, the steering torque T_(S) is controlled to be equalto the pinion torque T_(P), which provides a responsive feeling from asteady steering torque, similar to those obtained in the manualsteering. In other words, by controlling the steering torque T_(S) inaccordance with the vehicle speed VS, the responsive feeling from thesteering torque becomes light when the vehicle runs at lower speed, andsteady and stable when the vehicle runs at higher speed.

In addition, the electric power steering device 110 also includes anelectric motor drive circuit 23 configured to drive the electric motor4; a resolver 25; a torque sensor S_(T) configured to detect (measure) apinion torque T_(P) applied to the pinion shaft 7; a differentialamplifier 21 configured to amplify the output from the torque sensorS_(T); and the vehicle speed sensor S_(V) configured to detect (measure)a vehicle speed.

The electric motor steering control ECU 130 of the steering system 100has an electric power steering control part 130 a (which will bedescribed below; see FIG. 5) as a functional part of the electric powersteering device 110, which controls the driving of the electric motor 4.

The electric motor drive circuit 23 has switching elements, such asthree-phase FET bridge circuit, and is configured to generate asquare-wave voltage based on duty signals (DU, DV, DW) from the electricpower steering control part 130 a (see FIG. 5), to thereby drive theelectric motor 4.

The electric motor drive circuit 23 also has a function to detect(measure) a three-phase electric motor current I (IU, IV, IW) using aHall element (not shown).

The resolver 25 is configured to detect (measure) a rotation angle θ_(m)of the electric motor 4 and to output an angular signal θ, and examplesinclude a sensor for detecting a change in magnetoresistance which ispositioned in the vicinity of a magnetic rotor having a plurality ofrecess portions and projection portions arranged evenly along acircumference of the rotor.

The torque sensor S_(T) is configured to detect (measure) the piniontorque T_(P) applied to the pinion shaft 7. The torque sensor S_(T) isformed of magnetostrictive films adhered to the pinion shaft 7 at twodifferent positions along an axis thereof so as to exhibit oppositeanisotropies, and detection coils are arranged with a gap from thepinion shaft 7 along the surface (outer circumference) of the respectivemagnetostrictive films.

The differential amplifier 21 is configured to amplify a difference inpermeability change between two magnetostrictive films detected as aninductance change by the detection coil, and to output a torque signalT.

The vehicle speed sensor S_(V) is configured to detect (measure) thevehicle speed VS as a pulse number per unit time, and to output avehicle speed signal VS.

The functional configuration of the steering control ECU 130 will bedescribed later, together with the control by the electric powersteering device 110 and the control by the toe angle changers 120L,120R.

(Toe Angle Changer)

Next, a configuration of the toe angle changer will be described withreference to FIGS. 3 and 4.

FIG. 3 is a plain view of a toe angle changer on a left rear wheel side.FIG. 4 is a schematic cross sectional view showing a structure of anactuator of a toe angle changer.

The toe angle changers 120L, 120R are installed to the left rear wheel2L and the right rear wheel 2R of the vehicle, respectively. The toeangle changer 120L is taken as an example, and the left rear wheel 2L isshown in FIG. 3. The toe angle changer 120L includes the actuator 30 anda toe angle change controller (hereinbelow, referred to as “toe anglechange control ECU”) 37.

It should be noted that FIG. 3 shows the left rear wheel 2L only, butthe components are arranged in the same manner (symmetrically) on theright rear wheel 2R. It is also noted that the steering control ECU 130and the toe angle change control ECUs 37, 37 constitute the steeringcontroller of the present invention.

The cross member 12 extends substantially in the vehicle widthdirection, and end portions (in terms of the vehicle width direction)thereof are elastically supported by a rear side frame 11 of the vehiclebody. A trailing arm 13 extends substantially in the front-reardirection of the vehicle body, and a front end portion thereof issupported by a portion near the terminal (in terms of the vehicle widthdirection) of the cross member 12. The rear wheel 2L is fixed to a rearend portion of the trailing arm 13.

The trailing arm 13 is formed of a vehicle body-side arm 13 a attachedto the cross member 12, and a wheel-side arm 13 b fixed to the rearwheel 2L, which are connected to each other through a nearly verticalrotation axis 13 c. With this configuration, the trailing arm 13 isdisplaceable in the vehicle width direction.

With respect to the actuator 30, one end portion is attached through aball joint 16 to a front end portion of the wheel-side arm 13 b relativeto the rotation axis 13 c, and the other end (base end) portion of theactuator 30 is fixed to the cross member 12 through a ball joint 17.

As shown in FIG. 4, the actuator 30 includes an electric motor 31, adeceleration mechanism 33, a feed screw portion 35 and the like.

The electric motor 31 may be a brush motor, a brushless motor or thelike, which can rotate in both forward and reverse directions. Theelectric motor 31 has a temperature sensor 31 a configured to detect(measure) a winding temperature of a coil of the electric motor 31, andto input a detected temperature signal to a self-diagnostic part 81 d(see FIG. 8), which will be described below, of the toe angle changecontrol ECU 37.

The deceleration mechanism 33 is formed of, for example, doubleplanetary gears (not shown) or the like assembled therein. Herein, theself-diagnostic part 81 d and the temperature sensor 31 a constitute ananomaly detection unit of the present invention.

The feed screw portion 35 includes: a rod 35 a in a shape of a cylinder;a nut 35 c in a shape of a cylinder which has an internal thread 35 bformed in an inner periphery thereof and is inserted in the rod 35 a;and a screw shaft 35 d which engages with the internal thread 35 b andsupports the rod 35 a in such a manner that the rod 35 a is movable inan axial direction.

The feed screw portion 35, the deceleration mechanism 33 and theelectric motor 31 are encased in a case body 34 in an elongated cylindershape. To a portion of the case body 34 on a feed screw portion 35 side,a boot 36 is attached so as to cover both an end portion of the casebody 34 and an end portion of the rod 35 a, in order to prevent dust orforeign matter from attaching to an outer periphery of the rod 35 aexposed from the end portion of the case body 34, and to prevent dust,foreign matter or water from entering the case body 34.

One end portion of the deceleration mechanism 33 is connected to anoutput shaft of the electric motor 31, and the other end portion isconnected to the screw shaft 35 d. When the power of the electric motor31 is transmitted through the deceleration mechanism 33 to the screwshaft 35 d to rotate the screw shaft 35 d, the rod 35 a shifts in aright-left direction in the drawing (axial direction) relative to thecase body 34, and thus the actuator 30 contracts or expands. Due to thefrictional force caused by engagement of the screw shaft 35 d and theinternal thread 35 b of the nut 35 c, a toe angle of the rear wheel ismaintained constant, even when the electric motor 31 is not energizedand driven.

The actuator 30 also includes a stroke sensor 38 configured to detect(measure) the position of the rod 35 a (i.e., amount ofexpansion/contraction). In the stroke sensor 38, a magnet or the like isembedded so as to detect (measure) the location of the rod 35 a byutilizing magnetism. In this manner, by detecting the position of therod 35 a using the stroke sensor 38, the steering angles (toe angle) oftoe-in or toe-out of the rear wheels 2L, 2R are separately detected withhigh accuracy.

With the actuator 30 having the configuration as described above, theball joint 16 provided on an end portion of the rod 35 a is rotatablyconnected to the wheel-side arm 13 b of the trailing arm 13 (see FIG.3), and the ball joint 17 provided on the base end of the case body 34(right-hand end in FIG. 4) is rotatably connected to the cross member 12(see FIG. 3). When the power of the electric motor 31 rotates the screwshaft 35 d and the rod 35 a shifts leftward (in FIG. 4) (i.e., theactuator 30 expands), the wheel-side arm 13 b is pushed outward in thevehicle width direction (left direction in FIG. 3) to thereby leftwardturn the rear wheel 2L. On the other hand, when the rod 35 a shiftrightward (in FIG. 4) (i.e., the actuator 30 contracts), the wheel-sidearm 13 b is pulled inward in the vehicle width direction (rightdirection in FIG. 3) to thereby rightward turn the rear wheel 2L.

It should be noted that the position to which the ball joint 16 of theactuator 30 is attached is not limited to the wheel-side arm 13 b andthe actuator 30 can be attached to any position, such as on a knucklearm, as long as the toe angle of the rear wheel 2L can be changed. Inaddition, in the present embodiment, the toe angle changers 120L, 120Rare applied to an independent suspension system with semi-trailing arms.However, the present invention is not limited to this type of suspensionsystem, and may be applied to other types of suspension system.

For example, the actuator 30 may be introduced to a side rod of a doublewishbone type suspension, or a side rod of a strut type suspension.

In addition, the toe angle change control ECU 37 is unified with theactuator 30. The toe angle change control ECU 37 is fixed to the casebody 34 of the actuator 30, and connected to the stroke sensor 38 andthe temperature sensor 31 a through connectors or the like. Between twotoe angle change control ECUs 37, 37, and between the toe angle changecontrol ECU 37 and the steering control ECU 130, there are providedsignal circuits connecting them to each other.

To the toe angle change control ECU 37, power is supplied from a powersource (not shown), such as a battery, mounted on a vehicle. Also to thesteering control ECU 130 and the electric motor drive circuit 23, poweris supplied from a power source (not shown), such as battery, which isan independent system of that of the toe angle change control ECU 37.

(Steering Control ECU)

Next, functions of the steering control ECU will be described withreference to FIGS. 5 and 6.

FIG. 5 is a schematic diagram of a control function of a steeringcontrol ECU and toe angle changers in the steering system. FIGS. 6A and6B are graphs showing properties of a base signal computing part and adamper compensation signal computing part, respectively.

The steering control ECU 130 includes a microcomputer with components,such as CPU, ROM, RAM (not shown), and a peripheral circuit and thelike.

As shown in FIG. 5, the steering control ECU 130 includes: the electricpower steering control part 130 a configured to control the electricpower steering device 110 (see FIGS. 1 and 2); and a rear wheel toeangle control part 130 b, which is a feature of the present invention,configured to compute target toe angles of the rear wheel 2L, 2R and tocompute a compensation value for a difference in the self-aligningtorque (which will be described below) to be output to the electricpower steering control part 130 a.

(Electric Power Steering Control Part)

First, the electric power steering control part 130 a will be describedwith reference to FIGS. 5 and 6 (and FIG. 2 where appropriate).

The electric power steering control part 130 a includes: a base signalcomputing part (auxiliary torque calculating unit) 51; a dampercompensation signal computing part (auxiliary torque calculating unit)52; an inertia compensation signal computing part (auxiliary torquecalculating unit) 53; a Q-axis (torque axis) PI control part 54; aD-axis (axis of magnetic pole) PI control part 55; a 2-axis-to-3-phaseconversion part 56; a PWM conversion part 57; a 3-phase-to-2-axisconversion part 58; an electric motor speed calculating part 67; and anexciting current generation part 59.

The 3-phase-to-2-axis conversion part 58 converts a three-phase currentIU, IV, IW of the electric motor 4 detected by the electric motor drivecircuit 23 into a two-axis current, including a D-axis which is an axisof magnetic pole of the rotor of the electric motor 4, and a Q-axiswhich is obtained by electrically rotating the D-axis by 90 degrees. AQ-axis current IQ is proportional to the torque T_(M) generated at theelectric motor 4, and a D-axis current ID is proportional to an excitingcurrent. The electric motor speed calculating part 67 introduces adifferential operator to an angular signal θ of the electric motor 4, tothereby generate an angular velocity signal ω. The exciting currentgeneration part 59 generates a target signal for the exciting current ofthe electric motor 4, and if desired, field-weakening control can beperformed by making the D-axis current substantially equal to the Q-axiscurrent.

Based on the torque signal T and the vehicle speed signal VS, the basesignal computing part 51 generates a base signal (target value) D_(T) tobe used as a standard reference for a target signal IM₁ of the outputtorque T_(M)*. The signal is generated from a base table (first table)51 a with reference to the torque signal T and the vehicle speed signalVS, which table had been prepared by experimental measurement or thelike using a vehicle which is the same model as that of the presentembodiment but has only a front wheel steering function. FIG. 6A is agraph showing a function of the base signal D_(T), stored in the basetable 51 a. In the base signal computing part 51, a dead zone N1 isprovided where the base signal D_(T) is set to zero when the value ofthe torque signal T is small, and the base signal D_(T) linearlyincreases along a gain G1 when the value of the torque signal T islarger than the value in the dead zone N1. The base signal computingpart 51 increases the output along a gain G2 at specific torque values,and when the torque value further increases, the output is madesaturated.

In addition, a vehicle body in general has various road loads (roadreactions) depending on the running speed thereof. Accordingly, the gainis adjusted based on the vehicle speed signal VS. The load is heaviestduring a static steering (vehicle speed=0), and the load is relativelysmall at medium and low speeds. Therefore, when the vehicle speed VSbecomes higher, the base signal computing part 51 provides the driverwith road information with a larger manual steering zone, by making thegains (G1, G2) smaller and the dead zone N1 larger. In other words, inaccordance with the increase of the vehicle speed VS, a steadyresponsive feeling is provided from the steering torque T_(S). In thiscase, it is necessary that the inertia compensation be made also in themanual steering zone.

The base signal computing part 51 stores a backup table (second table)51 b, and in response to a command from a toe angle change controldiagnostic part 73, which will be described below, generates the basesignal D_(T) to be used as a standard reference for the target signalIM₁ of the output torque T_(M)*, from the backup table 51 b withreference to the torque signal T and the vehicle speed signal VS, whenthe toe angle changers 120L, 120R are in an abnormal state.

The backup table 51 b has a function of the torque signal T and thevehicle speed signal VS as also shown in FIG. 6A, but the values of thegains (G1, G2) are smaller by notable amounts than those in the case ofthe base table 51 a, for the same vehicle speed. With this setting, theauxiliary torque becomes smaller, making it easier for the driver tosense an abnormal state.

Referring to FIG. 5, the damper compensation signal computing part 52 isintroduced for compensating a viscosity in the steering unit, and forproviding a steering damper function for compensating convergence whenconvergence decreases during high-speed driving, by reading a dampertable 52 a with reference to the angular velocity signal co.

FIG. 6B is a graph showing a characteristic function of the damper table52 a, in which the line is formed of a several linear sections and acompensation value I as a whole increases as the angular velocity ω ofthe electric motor 4 increases. The graph is also characterized in thatthe compensation value I rapidly increases when the angular velocity ωis in a specific range. Moreover, as the vehicle speed signal VS becomeshigh, the gains are increased, the angular velocity of the electricmotor 4, i.e. the output torque T_(M)* of the electric motor 4 inaccordance with the speed of the steering wheel turn, decreases byincreasing the gain. To put it another way, when the steering wheel 3 isturned away from the home position, a current to the electric motor 4 isreduced; when the steering wheel 3 is returned to resume the homeposition, a large current is supplied to the electric motor 4. Forexample, when the steering wheel is further turned away and the angularvelocity ω becomes high, the angular velocity ω cannot be immediatelyreduced because of the inertia of the electric motor 4. In order toprevent this phenomenon, the damper compensation signal computing part52 makes the current supply to the electric motor 4 larger, to therebyperform an inhibitory control of the angular velocity co when thesteering wheel 3 is resuming the home position. Because of this steeringdamper effect, convergence of the steering wheel 3 is improved, tothereby stabilize the vehicle properties.

Referring to FIG. 5, an adder 61 is configured to subtract the outputsignal I of the damper compensation signal computing part 52 from theoutput signal D_(T) of the base signal computing part 51, and an adder62 is configured to add the output signal from the adder 61 and theoutput from the inertia compensation signal computing part 53 and tooutput the output signal IM₁.

It should be noted that an assist control is performed by a combinationof the base signal computing part 51, the damper compensation signalcomputing part 52 and the adder 61.

The inertia compensation signal computing part 53 is configured tocompensate an effect caused by the inertia in the steering unit, inwhich the torque signal T is computed from an inertia table 53 a.

In addition, the inertia compensation signal computing part 53compensates the lowering of the response caused by the inertia of therotor of the electric motor 4. To put it another way, when the rotationdirection of the electric motor 4 is made to be switched from forward toreverse or vice versa, it is difficult to immediately switch thedirection since the inertia tends to maintain the rotational state.Accordingly, the inertia compensation signal computing part 53 controlsthe timing of switching the rotation direction of the electric motor 4,so as to synchronize the timing of switching the rotation direction ofthe electric motor 4 with that of the steering wheel 3. In this manner,the inertia compensation signal computing part 53 reduces a response lagin the steering, which may otherwise be caused by inertia, viscosity orthe like in the steering unit, to thereby give an excellent steeringfeeling.

Further, the inertia compensation signal computing part 53 canpractically impart the above-mentioned features to various steeringproperties which varies depending on vehicle characteristics, such asthose specifically different among FF (Front engine Front wheel drive)vehicle, FR (Front engine Rear wheel drive) vehicle, RV (RecreationVehicle) and sedan (or saloon) car, and vehicle states, such as vehiclespeed, as well as road conditions.

The output signal IM₁ of the adder 62 is a target signal for the Q-axiscurrent which defines the torque of the electric motor 4.

An adder (difference compensation unit) 63 is configured to subtract,from the output signal IM₁, a compensation value for the difference inthe self-aligning torque which is output from a self-aligning torquecompensation computing part 72, details of which will be describedlater, and to send an output signal IM₂ to an adder 64.

The adder 64 is configured to subtract the Q-axis current IQ from theoutput signal IM₂, and to generate a deviation signal IE. The Q-axis(torque axis) PI control part 54 is configured to perform a P(proportional) control and an I (integral) control so as to reduce thedeviation signal IE.

An adder 65 is configured to subtract the D-axis current ID from theoutput signal of the exciting current generation part 59. The D-axis(axis of magnetic pole) PI control part 55 is configured to perform a PIfeedback control so as to reduce the output signal from the adder 65.

The 2-axis-to-3-phase conversion part 56 is configured to converttwo-axis signal including an output signal VQ from the Q-axis (torqueaxis) PI control part 54 and an output signal VD from the D-axis (axisof magnetic pole) PI control part 55 into three-phase signal UU, UV, UW.The PWM conversion part 57 is configured to generate duty signals (DU,DV, DW), which is a ON/OFF signal [PWM (Pulse Width Modulation) signal]having pulse widths proportional to the magnitude of the three-phasesignal UU, UV, UW.

It should be noted that the angular signal θ of the electric motor 4 isinput to the 2-axis-to-3-phase conversion part 56 and the PWM conversionpart 57, and a signal corresponding to the magnetic pole position of therotor is output.

(Rear Wheel Toe Angle Control Part)

Next, the rear wheel toe angle control part 130 b will be described withreference to FIGS. 5 and 7. As shown in FIG. 5, the rear wheel toe anglecontrol part 130 b includes a front wheel turning angle computing part68, a target toe angle computing part 71, the self-aligning torquecompensation computing part 72 and the toe angle change controldiagnostic part 73.

The front wheel turning angle computing part 68 is configured tocalculate a turning angle δ of the front wheels 1L, 1R based on theangular signal θ output from the resolver 25, and to input the result tothe target toe angle computing part 71 and the self-aligning torquecompensation computing part 72.

The target toe angle computing part 71 is configured to generate targettoe angles α_(TL), α_(TR) for respective rear wheels 2L, 2R, based onthe vehicle speed signal VS, a turning angle δ, and a turning angularvelocity which is obtained by differentiation of the turning angle δ(this can be easily obtained since the turning angle δ is proportionalto the angular velocity ω of the electric motor 4), and to input thetarget toe angles α_(TL), α_(TR) to the respective toe angle changecontrol ECUs 37, 37 configured to control respective toe angle changesof the left rear wheel 2L and the right rear wheel 2R (see FIG. 8). Thetarget toe angles α_(TL), α_(TR) are generated from the toe angle table71 a, with reference to the turning angle δ, the angular velocity δ′ ofthe turning angle δ and the vehicle speed VS, which table had beenprepared for each of the left rear wheel 2L and the right rear wheel 2Rin advance.

For example, the target toe angles or α_(TL), α_(TR) are defined by thefollowing formulae (4) and (5):

α_(TL) =K _(L)(VS,δ′,δ)·δ  (4)

α_(TR) =K _(R)(VS,δ′,δ)·δ  (5)

where each of K_(L)(VS), K_(R)(VS) represents a front-rear wheelsteering ratio which depends on the vehicle speed VS, the turning angleδ and the angular velocity δ′ of the turning angle. When the vehiclespeed is in a specific low-speed range, each of the target toe angles orα_(TL), α_(TR) of the rear wheel is generated in such a manner that therear wheels 2L, 2R are in antiphase relative to the front wheels, inaccordance with the turning angle δ, to allow the vehicle to turn in asmall radius.

In the high-speed range over the above-mentioned specific low-speedrange, when an absolute value of the angular velocity δ′ of the turningangle is a specific value or less, and at the same time, the turningangle δ is within a specific range (including right and left), thetarget toe angles or α_(TL), α_(TR) of the rear wheels 2L, 2R are set asthe same phase relative to the front wheels, in accordance with theturning angle δ. In other words, the target toe angles or α_(TL), α_(TR)of the rear wheels 2L, 2R are set so as to make the slip angle β smallduring lane change.

However, in the high-speed range over the above-mentioned specificlow-speed range, when the absolute value of the angular velocity δ′ ofthe turning angle exceeds a specific value, or when the turning angle δis too large to fall outside the specific range (including right andleft), the target toe angles or α_(TL), α_(TR) of the rear wheels areset to the antiphase relative to the front wheels, in accordance withthe turning angle δ.

It should be noted that, from the viewpoint of the stability in a turn,the target toe angles or α_(TL), α_(TR) generated in the target toeangle computing part 71 do not necessarily follow Ackerman-Jeantaudgeometry. Further, when the turning angle δ is 0°, each of the targettoe angles or α_(TL), α_(TR) may be, for example, 2°, with the wheelstoed in.

The detailed functions of the self-aligning torque compensationcomputing part 72 will be described with reference to FIG. 7 (and FIG. 1where appropriate). The self-aligning torque compensation computing part72 has a motion parameter calculating part (restoring torque calculatingunit or reference restoring torque calculating unit) 72 a, aself-aligning torque calculating part (restoring torque calculatingunit) 72 b, a reference self-aligning torque calculating part (referencerestoring torque calculating unit) 72 c, and a self-aligning torquedifference calculating part (difference compensation unit) 72 d.

The motion parameter calculating part 72 a is configured to calculate aslip angle β of the vehicle body, based on the yaw rate γ from the yawrate sensor S_(Y) (see FIG. 1), the vehicle speed VS, the lateralacceleration G_(S) from the lateral acceleration sensor S_(GS), theturning angle δ, and the target toe angles α_(TL), α_(TR) of the rearwheels 2L, 2R. The slip angle β is obtained from the slip angle table(not shown), with reference to the yaw rate γ, the vehicle speed VS, thelateral acceleration G_(S), the turning angle δ and the target toeangles or α_(TL), α_(TR) of the rear wheels 2L, 2R, which table had beenprepared by experimental measurement or the like in advance. The motionparameter calculating part 72 a also has a reference yaw rate conversiontable, a reference slip angle conversion table and a reference lateralacceleration conversion table (all are not shown). A reference yaw rateγ* corresponding to the present turning angle δ of the front wheels 1L,1R and the vehicle speed VS of the vehicle having only a front steeringwheel turn function is obtained from the reference yaw rate conversiontable with reference to the vehicle speed VS and the turning angle δ; areference lateral acceleration G*_(S) corresponding to the presentturning angle δ of the front wheels 1L, 1R and the vehicle speed VS ofthe vehicle having only a front steering wheel turn function is obtainedfrom the reference lateral acceleration conversion table; and areference slip angle β* of the vehicle having only a front steeringwheel turn function is calculated from the reference slip angleconversion table with reference to the reference yaw rate γ*, thevehicle speed VS, the reference lateral acceleration G*_(S) and theturning angle δ.

The calculated slip angle β is input to the self-aligning torquecalculating part 72 b, and the calculated reference yaw rate γ* and thereference slip angle β* are input to the reference self-aligning torquecalculating part 72 c.

The self-aligning torque calculating part 72 b calculates the restoringmoment T_(S, RTC) generated around the kingpin axis (or steering axis,not shown) of the front wheels 1L, 1R using, for example, the followingformula (6), and further converts the calculated value into theself-aligning torque T_(SAT, RTC) in terms of the torque around theshaft 3 c of the steering wheel 3 (see FIG. 2), as represented by thefollowing formula (7):

$\begin{matrix}{T_{s,{RTC}} = {2\xi \; {K_{f}\left( {\beta + {\frac{l_{f}}{VS}\gamma} - \delta} \right)}}} & (6) \\{T_{{SAT},{RTC}} = {C \cdot T_{S,{RTC}}}} & (7)\end{matrix}$

where ξ=ξc+ξn with the proviso that a caster trail is represented as ξcand a pneumatic trail is represented as ξn, l_(f) represents a distancebetween the front wheel axle and a center of gravity, K_(f) represents acornering stiffness of the front wheel tire, C represents a coefficientthat converts a moment around the kingpin axis (steering axis) into atorque around the shaft 3 c of the steering wheel 3.

In the same manner, the reference self-aligning torque calculating part72 c calculates the reference restoring moment T_(S)* generated aroundthe kingpin axis (or steering axis, not shown) of the front wheels 1L,1R in the case of a reference yaw rate γ*, the present turning angle δand the reference slip angle β* of a hypothetical vehicle having only afront wheel steering function, using, for example, the following formula(8), and further converts the calculated value into the referenceself-aligning torque T*_(SAT) in terms of the torque around the shaft 3c of the steering wheel 3 (see FIG. 2), as represented by the followingformula (9):

$\begin{matrix}{T_{S}^{*} = {2\xi \; {K_{f}\left( {\beta^{*} + {\frac{l_{f}}{VS}\gamma^{*}} - \delta} \right)}}} & (8) \\{T_{SAT}^{*} = {C \cdot T_{S}^{*}}} & (9)\end{matrix}$

The self-aligning torque difference calculating part 72 d calculates thecompensation value for the difference between the referenceself-aligning torque T*_(SAT) and the self-aligning torque in terms ofthe torque around the shaft 3 c (see FIG. 2) T_(SAT, RTC), calculatedabove with the formulae (7) and (9), by the following formula (10):

(T*_(SAT)−T_(SAT, RTC))×k  (10)

where k represents a coefficient and a tuning parameter.

The compensation value for the difference between the self-aligningtorques, calculated by the formula (10), is input to the adder 63.

It should be noted that the coefficient C may be a specific constantvalue, or a specific variable which changes depending on the vehiclespeed VS or the like. For example, when the base signal D_(T) isgenerated in the base signal computing part 51 where the gains G1, G2are changed in accordance with the torque signal T and the vehicle speedsignal VS, it would be convenient if the coefficient C is changed inaccordance with the torque signal T and the vehicle speed VS (i.e., thegain), since the same gain is used for the compensation of theself-aligning torque and that of the auxiliary torque.

In this manner, when the vehicle turns with a certain turning angle δ ofthe front wheels 1L, 1R, the compensation can be made for the differencebetween the self-aligning torque T_(SAT,RTC) of the all-wheel steeringvehicle and the self-aligning torque T*_(SAT) of the vehicle having asteering function of only front wheels 1L, 1R, in the adder 63 of theelectric power steering control part 130 a. Therefore, even when adriver who is used to a conventional vehicle having only a front wheelsteering function drives an all-wheel steering vehicle, the driver doesnot feel discomfort in the responsive feeling from the steering torqueof the steering wheel 3.

Next, the toe angle change control diagnostic part 73 will be described.The toe angle change control diagnostic part 73 is configured, whenreceives an anomaly detection signal from the self-diagnostic part 81 d(which will be described below, see FIG. 8) of the toe angle changecontrol ECU 37 in the toe angle changer 120L, 120R, to command theself-aligning torque compensation computing part 72 not to compute acompensation but to output a zero signal, and to output a command to thebase signal computing part 51 to switch the reference from the basetable 51 a to the backup table 51 b.

(Toe Angle Change Control ECU)

Next, the detailed configuration of the toe angle change control ECUwill be described with reference to FIG. 8. FIG. 8 is a blockconfiguration diagram showing a control function of a toe angle changecontrol ECU of a toe angle changer.

As shown in FIG. 8, the toe angle change control ECU 37 has a functionto drive control the actuator 30, and is formed of a control part 81 andan electric motor drive circuit 83. Each toe angle change control ECU 37is connected to the steering control ECU 130 through a communicationline, and also to the other toe angle change control ECU 37 through acommunication line.

The control part 81 includes a microcomputer with components, such asCPU, RAM, ROM, and a peripheral circuit, and has a target currentcalculating part 81 a, a motor control signal generation part 81 c andthe self-diagnostic part (anomaly detection unit) 81 d.

The target current calculating part 81 a of one toe angle change controlECU 37 (on aright rear wheel 2R side) is configured to calculate atarget current signal based on the target toe angle α_(TR) of the rearwheel 2R input through the communication line from the steering controlECU 130 and on the present toe angle α_(R) of the rear wheel 2R obtainedfrom the stroke sensor 38, and to output the result to the motor controlsignal generation part 81 c.

The target current calculating part 81 a of the other toe angle changecontrol ECU 37 (on a left rear wheel 2L side) is configured to calculatea target current signal based on the target toe angle α_(TL) of the rearwheel 2L input through the communication line from the steering controlECU 130 and on the present toe angle α_(L) of the rear wheel 2L obtainedfrom the stroke sensor 38, and to output the result to the motor controlsignal generation part 81 c.

Herein, the target current signal is a current signal required forsetting the actuator 30 so as to realize a desired operation amount ofthe actuator 30 (amount of expansion/contraction of the actuator 30 thatallows the rear wheel 2L (or 2R) to have a desired toe angle α_(TL) (orα_(TR))) at a desired speed.

In this manner, the target toe angle α_(TL) (or α_(TR)) is set promptlyin the target current calculating part 81 a, by feeding the present toeangle α_(L) (or α_(R)) and the target toe angle α_(TL) (or α_(TR)) andcorrecting the target current signal, and by feeding a change in thecurrent value required for the steering wheel turn of the rear wheel 2L(or 2R) which change is caused by the vehicle speed VS, road conditions,motional states of the vehicle, wear status of tire, or the like.

The motor control signal generation part 81 c is configured to receivethe target current signal from the target current calculating part 81 a,and to output the motor control signal to the electric motor drivecircuit 83. The motor control signal includes a value of the current tobe supplied to the electric motor 31, and a direction of the current.The electric motor drive circuit 83 is formed of, for example, a bridgecircuit with FET (Field Effect Transistor), and configured to supply anelectric motor current to the electric motor 31, based on the motorcontrol signal.

As shown in FIG. 8, the self-diagnostic part 81 d is configured todetermine whether or not an abnormal state is detected, based on aposition signal of the stroke sensor 38 of the toe angle changer 120L orthe toe angle changer 120R (to which the self-diagnostic part 81 d ofinterest belongs), a detection signal from a Hall element of theelectric motor drive circuit 83, a temperature signal from thetemperature sensor 31 a, and a state monitoring of the target currentcalculating part 81 a.

For example, the self-diagnostic part 81 d determines that a windingtemperature of the electric motor 31 is abnormal when the signal fromthe temperature sensor 31 a exceeds a specific value, and inputs aspecific target toe angle α_(SL) (or α_(SR)), such as 0°, to the targetcurrent calculating part 81 a. Herein, the target toe angles α_(SL) andα_(SR) are target toe angles regarding the left rear wheel 2L and theright rear wheel 2R, respectively, when anomaly is detected.

The self-diagnostic part 81 d is configured to monitor the detectionsignals from the target current calculating part 81 a and a Hall elementof the electric motor drive circuit 83, and to determine whether or notthe actuator 30 is locked, based on the position signal from the strokesensor 38: when it is determined that the actuator 30 is locked, theself-diagnostic part 81 d commands the electric motor drive circuit 83to stop the power supply to the electric motor 31, and inputs thepresent toe angle α_(L) (or α_(R)) as the target toe angle α_(SL) (orα_(SR)) to the target current calculating part 81 a, and then sends ananomaly detection signal and a signal of a mode indicating that aprocess is made in response to the anomaly detection, to theself-diagnostic part 81 d of the other toe angle change control ECU 37.

It should be noted that, for an anomaly detection unit, a watch dogcircuit may be provided as a peripheral circuit in addition to theself-diagnostic part 81 d, to monitor the control part 81. In this case,when an abnormal state of the control part 81 is detected, the electricmotor drive circuit 83 may be commanded to stop a power supply to theelectric motor 31, and then an anomaly detection signal may be output tothe self-diagnostic part 81 d of the other toe angle change control ECU37.

In addition, the self-diagnostic part 81 d of the toe angle changer 120L(or 120R) is configured to check whether or not there is an anomalydetection signal from the self-diagnostic part 81 d of the toe anglechange control ECU 37 of the other toe angle changer 120R (or 120L).When the anomaly detection signal is received, the target toe angleα_(SL) (or α_(SR)) is input to the target current calculating part 81 a,based on the signal of a mode indicating that a process is made.

In other words, the self-diagnostic part 81 d monitors a signalindicating whether or not the toe angle changer 120L (or 120R)corresponding to the toe angle change control ECU 37 of interest isnormally operated, and at the same time, monitors a signal indicatingwhether or not the toe angle changer 120R (or 120L) corresponding to theother toe angle change control ECU 37 is normally operated. When one ofthe toe angle changer 120 is found to be in an abnormal state, both ofthe toe angle change control ECUs 37, 37 perform a process in the samespecific mode.

Then, the self-diagnostic part 81 d sends the anomaly detection signalto the toe angle change control diagnostic part 73.

As described above, according to the present embodiment, even when adriver who is used to a vehicle having a steering function of only thefront wheels 1L, 1R drives a vehicle having the steering system 100 ofthe present embodiment, the difference between the referenceself-aligning torque T*_(SAT), based on the reference slip angle β* andthe reference yaw rate γ* in the vehicle having a steering function ofonly the front wheels 1L, 1R, and the present self-aligning torqueT_(SAT,RTC), based on the slip angle β and the reference yaw rate γ inthe all-wheel steering vehicle, which torques are obtained for the samevehicle speed VS and the same turning angle δ, is used as a compensationvalue for subtraction in the adder 63 from the target current IM₁ of theadder 62, and the obtained target current IM₂ is output as the auxiliarytorque to the electric motor 4. As a result, the driver can be providedwith a responsive feeling from the steering torque very similar to thatin a vehicle having only a front wheel steering function.

In addition, the base table 51 a to be referred to in the base signalcomputing part 51 may be not be formed separately for the vehicle towhich the steering system 100 is applied and for the vehicle having onlya front wheel steering function, and the same base table 51 a for theconventional vehicle having only a front wheel steering function may beused. In this case, the preparation of the control data in the basetable 51 a for the vehicle to which the steering system 100 is appliedcan be omitted.

For example, FIGS. 9-11 show changes over time of the yaw rate γ, theslip angle β, the lateral acceleration G_(S) and the moment around thekingpin axis (steering axis), with reference to the change in operationangle θ_(H) of the steering wheel 3, for both the vehicle to which thesteering system 100 is applied and the vehicle having only a front wheelsteering function, in the case where the operation angle θ_(H) of thesteering wheel 3 is operated up to 100° (deg) at the vehicle speed of 80km/h.

FIG. 9 shows graphs for a vehicle to which the steering system 100 isapplied: (a) shows a change over time of an operation angle θ_(H) (deg)of a steering wheel, (b) shows a change over time of a yaw rate γ(deg/s) of the vehicle, (c) shows a change over time of a slip angle β(deg) of the vehicle, and (d) shows a change over time of a lateralacceleration G_(S) (G).

FIG. 10 shows graphs for a vehicle having only a front wheel steeringfunction: (a) shows a change over time of an operation angle θ_(H) (deg)of a steering wheel, (b) shows a change over time of a yaw rate γ(deg/s) of the vehicle, (c) shows a change over time of a slip angle β(deg) of the vehicle, and (d) shows a change over time of a lateralacceleration G_(S) (G).

FIG. 11 shows a change over time of resorting moment of two front wheelsaround kingpin axis (steering axis), in accordance with a step input ofthe operation angle of the steering wheel θ_(H) of the steering wheel 3shown in FIGS. 9 and 10, with a comparison being made between thevehicle to which the steering system 100 is applied and a vehicle havingonly a front wheel steering function.

In the vehicle to which the steering system 100 is applied, the toeangles of the rear wheels 2L, 2R are set to antiphase relative to theturning angle δ of the front wheels 1L, 1R, and thus the vehicle canturn in a small radius, with a larger yaw rate γ, slip angle β andlateral acceleration G_(S), as compared with those in the vehicle havingonly a front steering wheel turn function. In accordance with this, therestoring moment around the kingpin axis (steering axis) becomes largerin the vehicle to which the steering system 100 is applied (curve a inFIG. 11) than in the vehicle having only a front wheel steering function(curve b). Therefore, in such a manner that a driver is provided with aresponsive feeling from the steering torque corresponding to such alarge restoring moment, the self-aligning torque compensation computingpart 72 calculates a compensation value corresponding to the differencein the restoring moment between the curve a and the curve b and theadder 63 makes subtraction based on the compensation value. With thisconfiguration, even when the base signal computing part 51 outputs atarget value of the auxiliary torque for the vehicle having only a frontwheel steering function corresponding to the curve b, the target signalof the auxiliary torque can be made appropriate for the vehicle havingall-wheel steering function, due to the processing in the self-aligningtorque compensation computing part 72 and the adder 63.

Further, when the toe angle change control diagnostic part 73 receivesan anomaly detection signal of the rear steering wheel turn functionfrom the toe angle change control ECU 37, the steering control ECU 130switches the table to be used in the base signal computing part 51 fromthe base table 51 a to the backup table 51 b, and sets the output signalfrom the self-aligning torque compensation computing part 72 to theadder 63 to a zero signal. With this configuration, the auxiliary torquebecomes smaller and the responsive feeling from the steering torquegiven to the driver becomes large, making it easier for the driver tosense an abnormal state of the steering function.

When the self-diagnostic part 81 d of one of the toe angle changecontrol ECUs 37,37 detects an abnormal state, the self-diagnostic part81 d sends an anomaly detection signal to the other toe angle changecontrol ECU 37, and both of the toe angle changers 120L, 120R arecontrolled so that the toe angles are fixed. Therefore, it is preventedthat a change of only one of the toe angles between the rear wheels 2L,2R remains controlled, and thus a driving performance is maintainedstable even when the toe angle changers 120L, 120R are in an abnormalstate.

It should be noted that, when an abnormal state of the rear wheel toeangle changers 120L, 120R is detected and the self-diagnostic part 81 dinputs a specific value (such as 0° or 0.5° toe-in) as the target toeangles α_(SL), α_(SR) for an abnormal state to the target currentcalculating part 81 a where the target toe angles or α_(TL), α_(TR) areset, the vehicle is limited to have only a front wheel steering functionlike a conventional vehicle, and based on the command from the toe anglechange diagnostic part 73, the self-aligning torque compensationcomputing part 72 outputs a zero signal as the compensation signal tothe adder 63. In this case, the base signal (target value) D_(T) outputfrom the base signal computing part 51 in accordance with the commandfrom the toe angle change diagnostic part 73 gives a larger responsivefeeling from the steering torque by referring to the backup table 51 a.Since the target signal of the auxiliary torque corresponding to thevehicle having only a front wheel steering function is output, thesteering properties coordinate with the vehicle properties, leading tothe prevention of a sudden shift in the change of the steering feelingthat the driver feels.

In the present embodiment, the toe angle change control ECU 37calculates the target current and is unified with the actuator 30 andthus separately arranged from the steering control ECU 130. With thisconfiguration, the detected value (position information) by the strokesensor 38 does not have to be sent to the steering control ECU 130, andit becomes possible to feed-back-wise process the position control andcurrent control in the toe angle change control ECU 37. As a result, anindependent feedback loop is formed in the toe angle changer 120L (or120R), and thus it becomes possible that settings can be made inaccordance with the individual actuator 30 in a different state fromthat of the other actuator 30 (i.e., it is not necessary to makesettings in accordance with the steering control ECU 130), leading toincrease in the processing speed. In other words, the steering controlECU 130 does not output a command including the actuation amount to thetoe angle change control ECU 37; instead, the steering control ECU 130outputs only a signal of the target toe angle α_(SL), α_(SR), resultingin a minimum load on the steering control ECU 130. Moreover, with thisconfiguration, it becomes easy to replace the toe angle change controlECU 37 to those having the electric motor drive circuit 83 correspondingto the actuator 30 having the steering effort specific to the type ofthat vehicle.

In addition, if the electric motor 31 of the actuator 30 is connected tothe steering control ECU 130, the feedback loop becomes significantlylong, which leads to a large phase lag, resulting in poor controlaccuracy. On the other hand in the present embodiment, the control part81 itself of the toe angle change control ECU 37 is configured tocalculate the target current, making the feedback loop shortest, thusimproving the control accuracy.

According to the steering system as described above, the responsivefeeling from the steering torque by the electric power steering deviceof the front wheel in the all-wheel steering vehicle is made nearlyequivalent to the responsive feeling from the steering torque by theelectric power steering device of the front wheel in the vehicle havingonly a front wheel steering, and the driver who is used to the vehiclehaving an electric power steering device only for front wheel does notfeel discomfort.

In addition, the steering system allows the driver to easily recognizethe abnormal state of the toe angle changer by the change in theresponsive feeling from the steering torque, when the toe angle changeris in an abnormal state, for example, the toe angle of the rear wheel islocked.

Modified Embodiment

The embodiment of the present invention has been described above.However, the present invention is not limited to the above embodiments,and it is a matter of course that the above embodiments may be properlymodified, for example, as described below.

(1) In the present embodiment, in order to obtain the turning angle δ ofthe front wheels 1L, 1R, the front wheel turning angle computing part 68makes calculation based on the angular signal θ of the electric motor 4.However, a turning angle sensor may be provided in the steering wheelturn mechanism to directly detect (measure) the turning angle δ.Alternatively, an operation angle sensor may be provided on the pinionshaft 7 and the turning angle δ may be computed from an operation anglesignal of the steering wheel 3.(2) In the self-aligning torque calculating part 72 b and the referenceself-aligning torque calculating part 72 c, the self-aligning torque andthe reference self-aligning torque around the shaft 3 c are calculatedusing the formulae (6)-(9). However, the torques can be calculated indifferent manners.

For example, a calculation model including the vehicle speed VS, turningangle δ, yaw rate γ and lateral acceleration G_(S) as input parametersmay be prepared from actual measurements using an actual vehicle inadvance, and the self-aligning torque and the reference self-aligningtorque may be calculated using the calculation model. This calculationmodel may be a nonlinear model, such as a neural network, or may be alinear model, such as a transfer function.

(3) The degree of change in the gain when switching from the base table51 a to the backup table 51 b may not be fixed, and based on thedifference between the value of the target toe angle α_(S) (α_(SL),α_(SR)) for the rear wheel in an abnormal state and the reference value(such as toe-in value of 0° or 0.5°), the assist amount may be madesmaller (i.e., the gains G1, G2 may be made smaller) when the differenceis larger.(4) In the electric power steering control part 130 a of theabove-described embodiment, the current in the electric motor 4 iscontrolled by setting the target current. Instead, a target voltage maybe set as a voltage to be applied to the electric motor 4.Alternatively, a target torque may be set as a torque to be output bythe electric motor 4, to thereby control the current in the electricmotor 4. Such a target voltage and a target torque are included in thetarget signal.

1. A steering system comprising: an electric power steering device whichcomprises a steering unit of front wheels having an electric motorconfigured to generate an auxiliary torque in accordance with at least asteering torque, and is configured to transmit the auxiliary torque tothe steering unit; toe angle changers capable of changing toe angles ofrespective right and left rear wheels in accordance with at least aturning angle of the front wheels and a vehicle speed; and a steeringcontroller configured to control the electric power steering device andthe toe angle changer, the steering controller comprising: an auxiliarytorque calculating unit configured to calculate a target value of theauxiliary torque and to output a target signal for driving the electricmotor, in which a difference between a first self-aligning torquegenerated at the front wheels and a second self-aligning torquegenerated at front wheels of a hypothetical vehicle having only a frontwheel steering function is compensated.
 2. The steering system accordingto claim 1, wherein the steering controller further comprises arestoring torque calculating unit configured to calculate the firstself-aligning torque based on at least a yaw rate, speed and slip angleof the vehicle and the turning angle of the front wheels, a referencerestoring torque calculating unit configured to calculate the secondself-aligning torque based on at least the vehicle speed and the turningangle of the front wheels, and a difference compensation unit configuredto calculate a difference between the first self-aligning torque and thesecond self-aligning torque and to compensate the target signal with thedifference.